This invention relates to laser Doppler velocimeters and in particular to laser Doppler velocimeters for measuring the velocity of wind or solid objects while compensating for motion of the laser Doppler velocimeter platform.
Conventional laser Doppler velocimeters (“LDVs”) transmit light to a target region (e.g., into the atmosphere) and receive a portion of that light after it has scattered or reflected from the target region or scatterers in the target region. This received light is processed by the LDV to obtain the Doppler frequency shift, fD. The LDV then conveys the velocity of the target relative to the LDV, v, by the relationship v=(0.5)cfD/ft where ft is the frequency of the transmitted light, and c is the speed of light in the medium between the LDV and the target.
LDV's are extremely useful and have a wide range of applications including, but not limited to: blood-flow measurements, speed-limit enforcement, spaceship navigation, projectile tracking, and air-speed measurement. In the latter case the target consists of aerosols (resulting in Mie scattering), or the air molecules themselves (resulting in Rayleigh scattering).
An example of a conventional air speed LDV 10 is illustrated in FIG. 1 and as disclosed in U.S. Pat. No. 5,272,513, the disclosure of which is incorporated herein by reference. The LDV 10 includes a source 20 of coherent light which may, if desired, be polarized. The source 20 projects a first coherent beam of light 30 into a beam shaper 40. The beam shaper 40 expands and collimates the beam 30 after which beam 30 enters a telescope 60. The telescope 60 projects the beam 30 in nearly collimated form into the target region 45.
The collimated beam strikes airborne scatterers (or air molecules) in the target region 45, resulting in a back-reflected or backscattered beam 50. A portion of the backscattered beam 50 is collected by the same telescope 60 which transmitted the beam 30, or to an adjacent receiver telescope (not shown). The case where the same telescope transmits and receives the light is known as a monostatic configuration, while the case of separate transmit and receive telescopes is known as a bistatic configuration. Monostatic configurations can only receive backscattered light. Bistatic configurations can be arranged to receive light that is substantially backscattered or at any other angle relative to the transmitted beam 30.
The light 50 collected by telescope 60 is then combined with a separate reference beam of light 70 in an optical mixer 80. An ideal optical mixer combines the two beams in such a way that they have the same polarization and occupy the same space, and directs the result onto a photodetector with a bandwidth sufficient to detect the measured Doppler frequency shift. The photodetector produces an electrical current 85 which includes a component whose frequency is the mathematical difference between the frequency of the reference beam 70 and the backscattered beam 50. The electrical current 85 is then analyzed by a signal processor 90 (e.g. electrical spectrum analyzer or a frequency counter) to determine the frequency difference and calculate the relative velocity component along the axis of the telescope 60 between the LDV 10 and the target region 45.
Ambiguities regarding whether the measured relative frequency is either positive or negative can be resolved by using the “in-phase and quadrature” detection method, as is known in the art. Another approach to resolving these ambiguities is to apply a stable, constant frequency shift either to the transmitted beam 30 or to the reference beam 70 (e.g. by using an acousto-optic cell). This creates an alternating current component in the electrical signal 85 with a frequency that is the sum of the constant frequency shift and the Doppler frequency shift, removing the directional ambiguity. An LDV wherein the frequency of the transmitted beam 30 and the frequency of the reference beam 70 are identical is said to use homodyne detection. Heterodyne detection is used when the frequencies of the transmitted beam 30 and reference beam 70 are different.
The reference beam 70 is selected to have a well-defined and stable optical frequency that bears a constant phase relationship with the transmitted beam 30. This is known as coherence. The requirement for coherence is easily achieved by using a laser as the source 20 and tapping the source 20 to create the reference beam 70 by means of an optical splitter (not shown).
Source 20 can be either a CO2, Nd:YAG, or Argon Ion laser (preferably lasing in the fundamental transverse mode and in a single longitudinal mode). However, air-speed targets (aerosols and/or molecules) generate very weak return signals compared to solid objects. Thus air-speed LDV's incorporating these laser sources that work over a range of thousands or even tens of meters require large amounts of laser power and are thus too large, bulky, heavy, fragile and possibly dangerous to be used in many desirable applications like air-speed determination for helicopters.
However, source 20 can also be a lightweight, low-cost, highly efficient, rare-earth-doped glass fiber (referred to hereafter as a fiber laser). Fiber lasers have several enormous advantages over other laser sources. Fiber lasers can be efficiently pumped by laser diodes whose emission wavelengths have been optimized for excitation of the rare-earth dopant. This makes the fiber lasers very energy efficient and compact, eliminating the need for cooling systems, flashlamps, and high current electrical sources. Moreover the glass fiber serves as a flexible waveguide for the light, eliminating the need for bulky optical components like mirrors and lenses that require rigid mechanical mounts in straight lines with stringent alignment tolerances. Fiber lasers are also more adaptable than solid-state lasers: the pulse repetition frequency (“PRF”) and pulse width in fiber lasers may be changed “on the fly,” while the PRF and pulse width in solid-state lasers are bound to narrow ranges or are even fixed.
Despite advances in conventional LDV's, improvements are still necessary. Sometimes it is desirable to locate the source laser 20 at a different, more accessible location than the telescope 60. For example, in a wind turbine generator (“WTG”) application the telescope can be located on the turbine, while its source laser and control electronics are best located in the nacelle or at the base of the tower that supports the WTG for ease of maintenance. In sailing applications the source is preferably located within the hull of the ship where it is protected from exposure to the elements.
These remote configurations can be made conveniently by using optical fiber to connect the source laser 20 and the telescope 60. Problems have occurred, however, in that the large optical power required for air speed measurements becomes limited by a non-linear effect that occurs in fiber optics known as stimulated Brillouin scattering (“SBS”). In fact, the longer a fiber optic is, the lower this limit becomes. The SBS power limit depends on other factors known to those skilled in the art, but it is a fundamental physical property of light traveling through transparent media and cannot be ignored.
In addition to the fiber laser-related shortcomings described above, it is also desirable to use an LDV with more than one telescope, and preferably three or more telescopes where all of the telescopes are transmitting a beam of light within the target region or regions simultaneously. The plurality of telescopes are each aimed at a different area of the target region, allowing for simultaneous velocity measurements along a plurality of different axes, thus allowing for a multi-dimensional velocity determination. Conventional LDVs for meteorological measurements or applications generally incorporate a single motorized telescope that takes measurements sequentially along different axes, or use three telescopes, switching from one to the next, and so on, to allow sequential measurements along the different axes. By simultaneously transmitting light to the different areas of the target regions, the accuracy of the readings is greatly improved while eliminating the need for any moving parts and the timeliness of the measurements is improved. It is also desirable that any such improvements to conventional LDVs still result in the use of eye-safe radiation sources, preferably in the 1.4-1.6 micron range.